Method for producing milling roll

ABSTRACT

A smooth roll (1) having a surface layer with water-retaining function is obtained by a method for producing a milling roll, comprising: a roughening process for blasting a surface (2a) of a roll substrate (2) constituting the smooth roll (1); and a coating process for spraying a thermal spray material over the roughened surface (2a) of the roll substrate (2) so as to form a thermal sprayed coating (3) having pores (5) that retain water. Satisfactory cereal powder is obtained by preventing drying of cereals in a milling process and retaining an adequate water content.

TECHNICAL FIELD

The present invention relates to methods for producing a milling rollused in a milling process of cereals such as wheat and the like.

BACKGROUND ART

Cereals such as wheat and the like are subjected to a milling processafter undergoing a selecting process, a tempering process and the like.Currently, milling of cereals such as wheat and the like is mainlycarried out by a roll crushing method. This system is said to have beendeveloped in the latter half of the 19th century, and therebyproductivity and quality are greatly improved as compared with a stonemilling system, contributing to industrialization of the millingindustry.

In the milling process according to the roll crushing method, firstly,the cereal is roughly broken by a brake roll to remove the skin, then,the operation of crushing and classifying with a smooth roll isrepeated, and finished as the final product. Generally, with respect tothe brake roll, the surface is subjected to a dressing process to formgrooves having a pitch of about 10 mm, and with respect to the smoothroll, a medium is inserted between the two rotating rolls and a matprocessing is performed on the surface by doing so, to form apearskin-like uneven surface. The mat processing for the smooth roll isdescribed in Patent Literatures 1 and 2.

For these milling rolls, chilled steel with a surface hardness in termsof Vickers hardness HV of about 600 is used. If milling rolls made ofchilled steel are used for a long period of time, unevenness present onthe surface will be worn out, and cereals will not be able to be crushedinto an appropriate size and shape, and powders having satisfactoryquality will not be obtained. In order to restore the surface roughnessof the milling roll, it is necessary to repair and process the surface,however, in the current method, polishing is performed first, then,repair and processing are performed, which is very troublesome and timeconsuming. Hence, it tends to be a case of exchanging the roll itself,which increases the cost.

Patent Literature 3 describes a food processing roll capable ofenhancing biting property of a raw material, capable of efficientlytreating the raw material and keeping the satisfactory biting propertyfor a long time. In this literature, a plurality of grooves are formedon the roll surface by laser processing, or an overlay welding layer isformed on the roll surface beforehand, and a plurality of concaveportions are formed on the surface by laser processing. A shape forbiting food is defined as a groove or a concave portion recessed fromthe smooth outer peripheral surface of the roll, so that even if wear ofthe roll is caused by long-term use, it is possible to keep the bitingproperty of the food.

CITATION LIST Patent Literature

[Patent Literature 1] Japanese Laid-Open Patent Publication No.H10-131948

[Patent Literature 2] Japanese Laid-Open Patent Publication No.H11-028621

[Patent Literature 3] Re-Published Patent Publication WO2013/179356

SUMMARY OF INVENTION Technical Problem

In the milling process of cereals, there are specific problems otherthan wear of the surface unevenness of the milling roll. For example,dried wheat is hard, scattered when crushed, and powder of the intendedshape and size cannot be obtained, so when crushing the wheat, a certainamount of water is contained. When the smooth roll described in PatentLiteratures 1 and 2 or the food processing roll described in PatentLiterature 3 is used for the milling process of wheat, the rollgradually heats up due to friction between the rolls through the wheatand friction between the roll and the wheat. When the heat is stored inthe roll, the water of the wheat is evaporated by the influence of thetemperature, and it is impossible to retain an appropriate water contentin the wheat and to obtain powder with a desired shape and size.

In view of the above-described problems of conventional technologies, anobject of the present invention is to provide a method for producing amilling roll capable of obtaining satisfactory cereal powder bypreventing drying of cereals in a milling process and keeping anappropriate water content.

Solution to Problem

The present inventors have investigated a means for preventing thedrying of cereals in the milling process and resultantly succeeded inretaining a suitable water content in cereals by thermally spraying thesurface of the milling roll to form a thermal sprayed coating havingpores to retain water, leading to solving of the problem.

That is, the method for producing a milling roll of the presentinvention is characterized by comprising: a roughening process forblasting a surface of a substrate; and a coating process for spraying athermal spray material over a roughened surface of the substrate so asto form a thermal sprayed coating having pores that retain water.

According to the present invention, since the thermal sprayed coating isformed on the surface of the substrate, even if the milling roll is usedfor a long period of time, unevenness of a surface layer can be kept andcrushing performance is not impaired. In addition, according to thethermal spraying, the size and presence ratio of pores in the coating tobe produced can be controlled by adjusting thermal spraying conditions.Since the thermal sprayed coating formed according to the presentinvention has pores for retaining water, heat can be hardly transferredto the cereal, and water retentivity in the surface layer of the millingroll can be enhanced, so that cooling property for the cereal can beobtained. In addition, since the thermal sprayed coating is formed onthe substrate having surface area and surface roughness increased byblasting, peeling of the thermal sprayed coating can be prevented.Hence, it is possible to eliminate the problems of wear on the surfacelayer of the milling roll and peeling of the thermal sprayed coating,and it becomes possible to retain the water content appropriate for thecereal, thus, satisfactory cereal powder can be obtained.

A surface roughness Ra of the milling roll suitable for crushing cerealsdiffers according to the purpose of use. When using the milling roll asa smooth roll, the surface roughness Ra after the coating process ispreferably 5-15 μm. On the other hand, when using the milling roll as abrake roll, the roughness like that of the smooth roll is not requiredsince crushing is possible by grooves inscribed on a substrate, and thesurface roughness Ra after the coating process is preferably 2-8 μm inview of durability.

Since thermal conductivity of ceramics and cermets is generally lower ascompared with metals, a milling roll produced by thermal spraying andcoating a ceramic or cermet material on the surface of a substrate madeof a metal material tends to accumulate friction heat generated during acrushing process. Hence, it is better not to set the thickness of thethermal sprayed coating formed by thermal spraying too large. On theother hand, however, unevenness having a surface roughness of a certainlevel or more is required for crushing of wheat and the like, and inorder to realize such unevenness by the thermal sprayed coating, acoating thickness of a certain level or more is required. This problemcan be solved if the surface roughness of the substrate is reflected inthe surface roughness of the milling roll. That is, the surfaceroughness Ra of the substrate is preferably adjusted within the range of−2 μm to +8 μm with respect to the surface roughness Ra of the millingroll after the coating process. Thus, it is possible to suppress thethickness of the thermal sprayed coating to minimum and to impartnecessary surface roughness to the milling roll.

Vickers hardness HV of the surface of the milling roll after the coatingprocess is preferably greater than 1000. Thus, wear resistance of themilling roll is markedly improved.

A thermal spray material is not limited, but in particular a carbidecermet is preferred. By using the carbide cermet as the thermal spraymaterial, a high surface hardness of the milling roll (specifically, avalue larger than 1000 in the Vickers hardness HV) can be easilyobtained and satisfactory wear resistance can be imparted.

An average thickness of the thermal sprayed coating is preferably 10-150μm. If the average thickness of the thermal sprayed coating is less than10 μm, the durability when used for a long period of time is concerned,whereas if it is more than 150 μm, the problem of accumulation offriction heat is concerned.

The method for producing a milling roll may include an adjusting processA for shot blasting as a post treatment after the coating process so asto smooth fine unevenness in undulation of a surface of the thermalsprayed coating. Thus, it is possible to prevent adhesion and cloggingof crushed cereals in the undulation of the surface of the thermalsprayed coating.

The method for producing a milling roll may include an adjusting processB for peak cutting as a post treatment after the coating process so asto planarize a tip of a convex portion on a surface of the thermalsprayed coating. Thus, the number of parts that make point contact withthe cereal on the surface of the thermal sprayed coating decreases, andthe wear resistance can be improved.

Advantageous Effects of Invention

According to the milling roll produced by utilizing the presentinvention, the cooling property for cereals can be obtained by thethermal sprayed coating having water-retaining function existing in thesurface layer. The thermal sprayed coating is formed on a substratehaving surface area and surface roughness increased by blasting, so thatpeeling of the thermal sprayed coating can be prevented. Due to the highcooling property of the thermal sprayed coating, cereal is preventedfrom getting heated in the milling process and drying of the cereal canbe suppressed. This makes it possible to retain an adequate watercontent in the cereal, and to obtain satisfactory cereal powder.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a smooth roll produced by the method forproducing a milling roll according to a first embodiment of the presentinvention.

FIG. 2 is an enlarged sectional view of a surface layer of the smoothroll in FIG. 1.

FIG. 3 is a graph of time until a surface temperature of each testmaterial reaches a predetermined temperature.

FIG. 4 is a schematic diagram of blasting in the roughening process.

FIG. 5(a) is an enlarged sectional view of a surface layer beforeconducting the shot blasting, and FIG. 5(b) is an enlarged sectionalview of a surface layer after conducting the shot blasting.

FIG. 6 is a sectional view of a surface layer before and afterconducting the peak cutting.

FIG. 7(a) is a perspective view of a part of a brake roll produced bythe method for producing a milling roll according to a second embodimentof the present invention, and FIG. 7(b) is an enlarged sectional view ofa surface portion of the brake roll.

FIG. 8 is an enlarged sectional view of a surface layer of a circledportion of the brake roll in FIG. 7.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described with reference tothe drawings. FIG. 1 is a perspective view of a smooth roll (millingroll) 1 produced by the method for producing a milling roll according tothe first embodiment of the present invention, and FIG. 2 is an enlargedsectional view of a surface layer of the smooth roll 1. In FIG. 1, twosmooth rolls 1, 1 are arranged in parallel. The smooth roll 1 of thepresent embodiment is provided with a roll substrate 2 (substrate)having a circular cross section, and a thermal sprayed coating 3 isformed on a surface 2 a of the roll substrate 2. The milling roll of thepresent invention is applied to rolls of all aspects used in the millingprocess, in addition to smooth rolls and brake rolls. In one example ofan actual milling process, cereals are supplied between rolls in whichthe smooth rolls 1, 1 shown in FIG. 1 are arranged side by side and arerotated inward in opposite directions to each other, and the cereals arecrushed and classified.

In the smooth roll 1 of this embodiment, any cereal can be milled.Specific examples of cereals include wheat, barley, oat, pearl barley,corn, rye, buckwheat, barnyard millet, foxtail millet, Chinese millet,sorghum, sorghum bicolor, macomo, and the like. These cereals may beused each singly or two or more of them may be used in combination.Application to wheat and barley, among these cereals, is particularlypreferred. Cereal flour derived from wheat includes hard flour,semi-hard flour, medium flour, soft flour, whole cereal flour, and durumsemolina.

The size and shape of each part such as a roll diameter and a length ofthe smooth roll 1 are not limited. A material constituting the rollsubstrate 2 on which the thermal sprayed coating 3 is formed may be anymaterial as long as it can be applied to a milling roll and is capableof forming various thermal sprayed coatings. As the materialconstituting the roll substrate 2, a metal material is suitably used.Specific examples of the metal material include metals selected from,for example, Fe, Cr, Ni, Mo, Co, Cu, Mn, Zn, Ta, W, Al, Ti, and Mg, oralloys such as chilled steel, stainless steel and the like, containingone or more of these metals. Such a metal material is formed byextrusion molding, cutting processing, plastic processing, forging, orthe like.

The thermal sprayed coating 3 is formed by collision of softened ormolten various thermal spray materials with the surface 2 a of the rollsubstrate 2 at a high speed and deposition of these materials thereon.The thermal spray material is not limited, but ceramics or cermetshaving high hardness are preferable.

Specific examples of ceramics as a thermal spray material includeoxide-based ceramics, nitride-based ceramics, carbide-based ceramics andboride-based ceramics, containing at least one element selected from thegroup consisting of Ni, Cr, Co, Al, Ta, Y, W, Nb, V, Ti, B, Si, Mo, Zr,Fe, Hf and La, and mixtures thereof.

The oxide-based ceramics include Al₂O₃, Cr₂O₃, HfO₂, La₂O₃, TiO₂, Y₂O₃,ZrO₂, Al₂O₃.SiO₂, NiO, ZrO₂.SiO₂, SiO₂, MgO, and CaO. The nitride-basedceramics include TiN, TaN, AlN, BN, Si₃N₄, HfN, NbN, YN, ZrN, Mg₃N₂, andCa₃N₂. The carbide-based ceramics include TiC, WC, TaC, B₄C, SiC, HfC,ZrC, VC, and Cr₃C₂. The boride-based ceramics include TiB₂, ZrB₂, HfB₂,VB₂, TaB₂, NbB₂, W₂B₅, CrB₂, and LaB₆.

Cermet materials which are composites of metal materials and ceramicmaterials may be used as the thermal spray material. The cermetmaterials include a composite material of the ceramic material selectedfrom the group consisting of Cr₃C₂, TaC, WC, NbC, VC, TiC, B₄C, SiC,CrB₂, WB, MoB, ZrB₂, TiB₂, FeB₂, CrN, Cr₂N, TaN, NbN, VN, TiN and BN,and the metal material selected from the group consisting of Ni, Cr, Co,Al, Ta, Y, W, Nb, V, Ti, B, Si, Mo, Zr, Fe, Hf and La; and the like.Among them, carbide cermets are particularly preferable because acoating with high hardness is easily obtained.

The surface of the thermal sprayed coating 3 is not sealed and a largenumber of pores 5 are present inside the thermal sprayed coating 3. Byretaining water inside these pores 5, the thermal sprayed coating 3 hashigh water retentivity. Thus, water retentivity is given to the surfacelayer of the smooth roll 1, and the cereal is prevented from gettingheated in the milling process, and simultaneously, cooling property forthe cereal is obtained and drying of the cereal can be suppressed. Then,the cereal is crushed by the surface unevenness of the smooth roll 1while retaining an appropriate water content in the cereal, wherebysatisfactory cereal powder can be obtained.

An average porosity of the thermal sprayed coating 3 may be about0.5-15%, and preferably 2.0-10%. Adjustment of the average porosity ismade by selection of thermal spraying methods and thermal sprayingconditions. The average porosity at which the pores 5 inside the thermalsprayed coating 3 can well retain water is 2.0% or more. However, if theaverage porosity is increased, although the water retentivity increases,there is a concern that the wear resistance will decrease. Hence, fromthe viewpoint of maintaining the wear resistance, the average porosityis preferably 10% or less.

The following methods are effective for adjusting the porosity inthermal spraying construction. That is, effective are selection ofparticle diameter of the thermal spray material, adjustment of particlevelocity during thermal spraying, and adjustment of spraying distance,which are used for coating formation. For example, in the case where theporosity of the thermal sprayed coating is reduced to form a densecoating, it is preferable to use powder having a fine particle diameteras the thermal spray material, and furthermore, it is preferable thatthe particle velocity is higher. It is also possible to select a methodfor setting the thermal spraying distance to be short so that heat fluxis obtained at which the coating does not break due to heat effect. Onthe other hand, in order to increase the porosity and form a porouscoating, forming of a coating by a method opposite to the above methodis effective. However, the above adjustments need to be changed only tothe extent that does not impair other coating properties such ashardness, wear resistance, and surface shape retention.

In order to verify heat transfer characteristics based on the watercontent of the thermal sprayed coating, the following experiment wasconducted. For the experiment, two samples of each of the following testmaterials A to C and one sample of the following test material D werefirstly prepared.

Substrate: Stainless steel materialSubstrate size: 5 cm square, 5 mm thicknessBlasting conditions: Alumina particles (#60), 0.4 MPa pressureThermal spray material: WC—CrNiThermal spraying method: HVOF

Porosity: 2-4%

Cross section was observed by using SEM with 200 times. There wasvariation depending on measurement place. From sectional SEM-BEI image,black parts inside the coating were considered as pores and the ratio ofthe pores to the entire coating was calculated. Thermal sprayed coatingthickness:

50 μm (test materials A1, A2)

100 μm (test materials B1, B2)

150 μm (test materials C1, C2)

No thermal sprayed coating (test material D)

Tap water was dropped on the surface of each of test materials A1, B1,and C1, and the thermal sprayed coating was sufficiently moistened. Thewater remaining on the surface was lightly wiped off with Kimtowel. Thetest materials A1 to C1 containing water were placed on a plate heaterkept at 100° C., and the surface temperature of each test material wasmeasured with a contact thermometer. The time until the surfacetemperature reached 40° C., 60° C., and 80° C. was measured andrecorded.

The test materials A2, B2, C2, and D were placed on the plate heaterkept at 100° C., and the surface temperature of each test material wasmeasured with the contact thermometer. The time until the surfacetemperature reached 40° C., 60° C., and 80° C. was measured andrecorded.

From these measurement records, heat transfer characteristics based onthe coating thickness difference and the water content difference wereverified. As shown in Table 1 and FIG. 3, there was hardly anydifference in temperature rise up to 40° C. regardless of the presenceor absence of water in any of the samples. However, the thermal sprayedcoating containing water reached the target temperature in a longer timeat 80° C. From the above, it was found that the temperature rise can bemore suppressed for the thermal sprayed coating containing water,regardless of the coating thickness.

TABLE 1 Test material Presence or absence Before test Reached time (sec)No of water (24.4° C.) 40° C. 60° C. 80° C. A1 Presence 0 19 116 223 A2Absence 0 14 96 212 B1 Presence 0 15 99 222 B2 Absence 0 15 94 200 C1Presence 0 16 110 240 C2 Absence 0 18 105 214 D Absence 0 15 100 182

An average thickness t of the thermal sprayed coating 3 is appropriatelyset, and is preferably 10-150 μm, more preferably 20-100 μm. When theaverage thickness t of the thermal sprayed coating 3 is too small, thedurability is concerned, while when too large, heat amount to beaccumulated in the roll substrate 2 is increased due to frictional heat,to promote drying of the cereals in the milling process. Especially,since ceramics and cermets generally have lower thermal conductivity ascompared with metals, when ceramics or cermets are thermally sprayed onmetallic materials, this point should be noted.

The surface roughness Ra of the roll substrate 2 is adjusted so as to bein the range of −2 μm to +8 μm with respect to the surface roughness Raof the smooth roll 1 finally aimed at. In other words, the surfaceroughness Ra of the smooth roll 1 reflects the surface roughness Ra ofthe roll substrate 2. The thermal sprayed coating 3 of the presentembodiment is formed so as to have uniform thickness t so that thesurface roughness Ra of the roll substrate 2 is reflected in the surfaceroughness Ra of the smooth roll 1. Here, the “uniform” means that themaximum thickness and the minimum thickness of the same coating areincluded within ±30% of the average thickness, respectively.

Since the thermal sprayed coating 3 is formed on the surface 2 a of theroll substrate 2 having surface area and surface roughness increased byadjusting the surface roughness, peeling of the thermal sprayed coating3 can be prevented. An undercoat layer may be provided between the rollsubstrate 2 and the thermal sprayed coating 3 for the purpose ofimproving adhesion and the like.

The surface roughness Ra of the smooth roll 1 is set to 5-15 μm from theviewpoint of crushing and classification of cereals. Thus, it ispossible to obtain satisfactory cereal powder. The surface hardness ofthe surface 1 a of the smooth roll 1 is high, and the surface hardnessthereof is adjusted to a value greater than 1000 in terms of Vickershardness HV. Thereby, the wear resistance of the smooth roll 1 can beimproved.

An example of a method for producing the smooth roll 1 in which thethermal sprayed coating 3 is formed on the surface will be described.There are conducted in this order, a roughening process for rougheningthe surface 2 a of the roll substrate 2, a cleaning treatment of thesurface 2 a of the roll substrate 2, and a coating process for sprayingthe thermal spray material onto the roll substrate 2 to form the thermalsprayed coating 3 having the pores 5 for retaining water. Otherprocesses such as a preheating process and the like may be includeddepending on the type of the substrate and the kind of the thermal spraymaterial.

In the roughening process, unevenness is formed on the surface 2 a ofthe roll substrate 2 by blasting for causing a shot material 21 tocollide with the surface 2 a of the roll substrate 2 from a blast nozzle20, as shown in FIG. 4. The “blasting” refers to a technique forspraying particles (grit) having a rough surface onto a surface of asubstrate with compressed air or the like to roughen the surface of thesubstrate. The roughness of the surface 2 a of the roll substrate 2 iswithin the range of −2 μm to +8 μm with respect to the surface roughnessRa of a milling roll after the coating process as a post process. Asprocessing conditions, the kind and grain size of a blast material,injection pressure, blasting time, and the like are appropriately set.The blasting can be carried out one by one against the roll substrates.In conventional techniques, a medium is poured into between two rollsrotating in opposite directions to each other, and a similar surfaceroughness is imparted to each of a pair of rolls. However, according tothe blasting, since the surface roughness Ra of individual milling rollcan be uniquely defined, it is possible to flexibly change the designaccording to the purpose of use, for example, by making the surfaceroughness Ra different between a pair of milling rolls, and the like.

A thermal spraying method for obtaining the thermal sprayed coating 3 inthe coating process includes an atmospheric plasma thermal sprayingmethod, a low-pressure plasma thermal spraying method, a high-speedflame thermal spraying method, a gas flame thermal spraying method, anarc thermal spraying method, a detonation thermal spraying method, andthe like. In order to make it possible for the thermal sprayed coating 3to retain water, these thermal spraying methods are appropriatelyselected, and further appropriately set according to the thermalspraying method are thermal spraying conditions such as the kind of thethermal spray material, heat source temperature, thermal spraying angle,thermal spraying distance, and the like.

In the case of thermal spraying the cermets, the high-speed flamethermal spraying method (HVOF) is particularly suitable. This thermalspraying method is a thermal spraying method using combustion energy ofcombustion gas as a heat source. By raising pressure in a combustionchamber, high-speed flame comparable to explosive combustion flame isgenerated, and a thermal spray material is supplied to the center ofthis combustion flame jet. The thermal spray material is accelerated,molten or semi-molten, and continuously sprayed at a high speed. Sincethe molten thermal spray particles collide with the substrate atsupersonic speed, the thermal sprayed coating 3 having high adhesion canbe formed. For fuels used as the heat source, used are kerosene, and ascombustion gas, acetylene, ethylene, propane and the like, which aremainly composed of carbon and hydrogen, in addition to H₂ gas.

In the case of thermal spraying the ceramics, the plasma thermalspraying methods are particularly suitable. The plasma thermal sprayingmethod is a thermal spraying method in which a thermal spray material isheated by plasma and molten into liquid thermal spray particles, and thethermal spray particles are caused to collide with coating-formingsurface of the substrate at high speed by plasma jet. The plasma thermalspraying method using electric energy as a heat source is a method inwhich a coating is formed using argon, hydrogen, nitrogen, and the likeas a source of plasma. Since the method has a high heat sourcetemperature and a high flame rate, it is possible to form a coatinghaving a high melting point.

As described above, after the thermal sprayed coating 3 is formed on thesurface 2 a of the roll substrate 2, an adjustment process for adjustingthe surface shape of the thermal sprayed coating 3 may be performed. Asa result, it is possible to obtain surface properties according to theintended use.

After the thermal sprayed coating 3 is formed, the shot blasting ispreferably carried out (adjusting process A). The “shot blasting” refersto a technique for spraying spherical particles onto the surface of asubstrate with compressed air or the like to adjust the surface shape ofthe substrate. In the undulation on the surface of the thermal sprayedcoating 3, fine unevenness as shown in FIG. 5(a) is present. By the shotblasting, the fine unevenness in the undulation on the surface of thethermal sprayed coating 3 can be smoothed, and it is possible to preventadhesion and clogging of the crushed cereal in the undulation, as shownin FIG. 5(b).

After the thermal sprayed coating 3 is formed, the peak cutting may becarried out as necessary (adjusting process B). As a method for peakcutting, buffing or the like can be exemplified. By the peak cutting,the tip of the convex portion on the surface of the thermal sprayedcoating 3 can be planarized as shown in FIG. 6, and the wear resistancecan be improved. The peak cutting may be carried out after conductingthe shot blasting.

Such shot blasting and peak cutting can be carried out one by oneagainst the milling rolls. As a result, since the surface shape of eachmilling roll can be adjusted uniquely, the design can be flexiblychanged according to the purpose of use. Depending on the peak cutting,the surface roughness Ra of the thermal sprayed coating 3 will change.However, it is preferable to keep the surface roughness Ra within thenumerical range as described above even after the peak cutting so thatcrushing performance for the cereal is not imparted.

According to the method for producing a milling roll of the presentembodiment, since the thermal sprayed coating 3 is formed on the surface2 a of the roll substrate 2 by the thermal spraying, unevenness of thesurface layer can be kept even if the smooth roll 1 is used for a longperiod of time and the crushing performance is not impaired. Inaddition, according to the thermal spraying, it is possible to controlthe size and presence ratio of the pores 5 in the coating to be formedby adjusting the thermal spraying conditions.

Since the thermal sprayed coating 3 formed by this method for producinga milling roll has pores 5 for retaining water, it is possible to makeit hard to transmit heat to cereals and the water retentivity in thesurface layer of the smooth roll 1 is enhanced, so that the coolingproperty for the cereals can be obtained. Further, since the thermalsprayed coating 3 is formed by thermal spraying on the roll substrate 2having surface area and surface roughness increased by the blasting,peeling of the thermal sprayed coating 3 can be prevented. Hence, it ispossible to eliminate the problems of wear on the surface layer of thesmooth roll 1 and the peeling of the thermal sprayed coating 3, and thewater retentivity is improved and the cooling property for the cerealsis enhanced. This high cooling property prevents the cereals fromgetting heated in the milling process, and it can suppress drying of thecereals. This makes it possible to retain an adequate water content inthe cereals, and to obtain satisfactory cereal powder.

The method for producing a milling roll of the above embodiment is anexample and is not restrictive. In the first embodiment, the method forproducing a milling roll was applied to a smooth roll, but the methodmay be also applied to a brake roll. FIG. 7(a) is a perspective view ofa part of a brake roll 10 produced by the method for producing a millingroll according to a second embodiment of the present invention. FIG.7(b) is an enlarged sectional view of the surface portion of the brakeroll 10. On the surface portion of the brake roll 10, a plurality ofgrooves 11 extending axially are formed over the entire circumference bymachining. Each groove 11 has a V-shaped cross section consisting of twoinclined surfaces having different inclination angles, and a top 12 asthe outer periphery is present between the adjacent grooves 11, 11.

FIG. 8 is an enlarged sectional view of the surface layer of the circledportion M of the brake roll 10 in FIG. 7(b). A thermal sprayed coating14 is formed on a surface 13 a of a roll substrate 13 of the brake roll10. Appropriately set within the above-described ranges according tofunction of the brake roll are materials constituting the roll substrate13, thermal spray materials for forming the thermal sprayed coating 14,average porosity of a lot of pores 15 present inside the thermal sprayedcoating 14, adjustment of the porosity in thermal spray construction,and thickness t of the thermal sprayed coating 14.

Also appropriately set within the above-described ranges are surfacehardness of the surface 10 a of the brake roll 10, and adjustment ofsurface roughness Ra of the roll substrate 13 to be within the range of−2 μm to +8 μm with respect to surface roughness Ra of the brake roll 10finally aimed at. The surface roughness Ra of the brake roll 10 is setto 2-8 μm from the viewpoint of removal of the skin of cereals, which isa pre-process of the process using a smooth roll.

The method for producing the brake roll 10 in which the thermal sprayedcoating 14 is formed on the surface thereof is the same as in the firstembodiment. That is, carried out in this order are a roughening processfor roughening the surface 13 a of the roll substrate 13, a cleaningtreatment of the surface 13 a of the roll substrate 13, and a coatingprocess for spraying a thermal spray material on the roll substrate 13to form the thermal sprayed coating 14 having pores 15 for retainingwater. Other processes such as a preheating process and the like may beincluded depending on the type of the substrate and the kind of thethermal spray material. Also contained in the processes of thisembodiment can be adjusting processes A and B for adjusting surfaceshape of the thermal sprayed coating 14 after the thermal sprayedcoating 14 is formed on the surface 13 a of the roll substrate 13.

Depending on specification and construction embodiment of the millingroll, other processes may be included in the method for producing themilling roll. The configurations and processes described in the aboveembodiments can be changed as long as effects of the present inventionare not impaired, and other configurations and processes to be providedas necessary are not limited. The scope of the present invention isdefined by the claims, and it is intended that all modifications withinmeaning and scope equivalent to the claims are included.

DESCRIPTION OF REFERENCE CHARACTERS

-   -   1 Smooth roll    -   1 a Surface of smooth roll    -   2 Roll substrate    -   2 a Surface of roll substrate    -   3 Thermal sprayed coating    -   5 Pores    -   t Thickness    -   10 Brake roll    -   10 a Surface of brake roll    -   11 Groove    -   12 Top    -   13 Roll substrate    -   13 a Surface of roll substrate    -   14 Thermal sprayed coating    -   15 Pores    -   20 Blast nozzle    -   21 Shot material

1. A method for producing a milling roll, comprising: a rougheningprocess for blasting a surface of a substrate; and a coating process forspraying a thermal spray material over a roughened surface of thesubstrate so as to form a thermal sprayed coating having pores thatretain water.
 2. The method for producing a milling roll according toclaim 1, wherein a surface roughness Ra of the milling roll after thecoating process is 2-15 μm.
 3. The method for producing a milling rollaccording to claim 1, wherein a surface roughness Ra of the substrateafter the roughening process is within the range of −2 μm to +8 μm withrespect to a surface roughness Ra of the milling roll after the coatingprocess.
 4. The method for producing a milling roll according to claim1, wherein Vickers hardness HV of a surface of the milling roll afterthe coating process is greater than
 1000. 5. The method for producing amilling roll according to claim 1, wherein the thermal spray material isa carbide cermet.
 6. The method for producing a milling roll accordingto claim 1, wherein an average thickness of the thermal sprayed coatingis 10-150 μm.
 7. The method for producing a milling roll according toclaim 1, comprising: an adjusting process A for shot blasting after thecoating process so as to smooth fine unevenness in undulation of asurface of the thermal sprayed coating.
 8. The method for producing amilling roll according to claim 1, comprising: an adjusting process Bfor peak cutting after the coating process so as to planarize a tip of aconvex portion on a surface of the thermal sprayed coating.